The present invention relates to video processing, and more particularly to a multi-standard video decoder having improved performance for standard and non-standard composite video signals.
Color video information is often transmitted and stored as a composite (or CVBS) signal that includes a luminance component (Y), a chrominance component (C), a blanking signal, and vertical and horizontal synchronization signals. The luminance component expresses the intensity (i.e., black to white) of a picture, and the chrominance component expresses color and its intensity. The chrominance component is generated by modulating a subcarrier signal with two color difference components (either U and V, or I and Q, depending on the color format being used). The phase and amplitude of the modulated signal determines the color and the intensity, respectively. The chrominance component is then added to, or superimposed over, the luminance component to generate the xe2x80x9cactivexe2x80x9d portion of the composite signal.
A composite video signal typically conforms to one of several standards, such as NTSC (National Television Systems Committee), PAL (phase alternation each line), or SECAM (sequential color with memory) standards. Each video standard covers particular geographic regions, with the NTSC standard being used in the United States and Japan, the PAL standard being used in South America and Europe, and the SECAM standard also being used in Europe. Each of these standards defines the format and characteristics of the composite video signal, such as the line (or horizontal) frequency and duration, the color subcarrier frequency, the color modulation format, and so on. Each standard also defines the relationship between the color subcarrier frequency (fSC) and the line frequency (fH). For a video signal that conforms to a standard, the line frequency can be determined if the color subcarrier frequency is known, and vice versa.
Some common video signals do not exactly conform to the video standard. For example, a video signal from a VCR includes general NTSC characteristics, but can have line duration that varies from line to line. This may be due to, for example, imprecision in the mechanical arrangements used to advance the tape during recording or playback. In addition, the color subcarrier frequency in a VCR video signal is typically not related to the line frequency, as specified by the video standards.
The composite signal format facilitates transmission and storage of video information. The composite signal is decomposed or decoded into a (Y, U, V) or (R, G, B) format suitable for display on a television or monitor. These output formats are known in the art.
For a composite signal that conforms to a standard (e.g., NTSC), digital decoding is conventionally achieved by first sampling the composite video signal with a clock signal from a clock source. The clock source is typically locked to either the line frequency (i.e., for a line-locked decoder architecture) or the subcarrier frequency (i.e., for a burst-locked decoder architecture) of the video signal with a phase lock loop (PLL). The sampling frequency is selected to be kxc2x7fH for the line-locked architecture and mxc2x7fSC for the burst-locked architecture. For an NTSC-compliant signal, k is typically 910 and m is typically 4.
Conventional video decoders typically employ either the line-lock or burst-lock architecture for decoding the composite video signal. The burst-lock architecture can allow for a simplified color demodulator design, since the video signal is sampled at m times the color subcarrier frequency. However, the burst-lock architecture is not optimal for some applications. For example, for non-standard video signals such as those from VCRs, the burst-lock architecture can generate different number of samples for each horizontal line, which can cause misalignment in the decoded picture. Also, since each standard specifies a different subcarrier frequency, the burst-lock architecture typically requires multiple crystals to support multiple standards.
The line-lock architecture can generate a fixed number of samples for each video line, which solves the problem of picture misalignment for non-standard video signals. However, since the sampling frequency is not locked to the color subcarrier, color demodulation is typically more complicated.
Accordingly, video decoding techniques that provide improved performance for standard and non-standard composite video signals are highly desirable. It is also desirable that these techniques support different video standards and are (relatively) simple to implement.
The invention provides video decoding techniques that provide improved picture quality for standard and non-standard video signals. For improved color separation (i.e., demodulation) of non-standard video signals, a comb filter having a variable delay can be used. To provide a properly aligned decoded picture with a non-standard video input signal, the decoded output components can be generated with a time offset, as described below. The timing and resampling signals can be provided by one or more PLLs having multiple operating modes, with each mode having particular loop characteristics and better suited for some input signal conditions. The video signal can also be resampled with a burst-locked front end and resampled again with a line-locked back end.
An aspect of the invention provides a video decoder for decoding a composite video signal. The video decoder includes an analog-to-digital converter (ADC), an input resampler, and a Y/C separator, all coupled in series. The ADC receives and digitizes the composite video signal to generate ADC samples. The input resampler receives and resamples the ADC samples with a first resampling signal to generate resampled video samples. The YIC separator receives and separates the resampled video samples into luminance and chrominance components. The Y/C separator includes a delay element that receives the resampled video samples and provides a variable amount of delay. For improved performance, the Y/C separator can be implemented with an adaptive comb filter.
The variable amount of delay can be adjustable from video line to video line, and is typically based on an approximated duration of the video line. For an NTSC signal, the variable delay is select to be (mxc2x7n+m/2) samples, where m is a ratio of frequencies of the first resampling signal and a color subcarrier of the composite video signal, and n is an integer selected such that (mxc2x7n+m/2) samples most approximate the duration of the video line. For a PAL signal, the variable delay is selected to be (mxc2x7n+3m/4) samples. For ease of color demodulation, the first resampling signal is locked to the color bursts of the composite video signal and has a frequency that is four times that of the color bursts.
The video decoder can further include a color demodulator that receives and demodulates the chrominance component from the Y/C separator into color difference components. The video decoder can further include an output resampler that receives and resamples the luminance and color difference components with a second resampling signal to generate output video components. The video decoder typically includes additional circuitry for providing the required timing and resampling signals.
Another aspect of the invention provides a video decoder for decoding a composite video signal. The video decoder includes an input sampling circuit, a color decoder, and a skew compensation circuit, all coupled in series, and a timing circuit coupled to the input sampling circuit. The input sampling circuit receives and digitizes the composite video signal to generate video samples. The color decoder receives and decodes the video samples to generate decoded video components. The timing circuit receives the video samples and generates a control signal indicative of an approximated time difference between the start of a video line and a burst phase of the video line. The skew compensation circuit receives the decoded video components and the control signal and generates output video components having a time offset based on the time difference indicated by the control signal.
In an embodiment, the input sampling circuit includes an ADC coupled to an input resampler. The ADC receives and digitizes the composite video signal to generate ADC samples. The input resampler receives and resamples the ADC samples with a first resampling signal to generate the video samples. For ease of color demodulation, the first sampling signal is locked to the color burst of the composite video signal. In an embodiment, the color decoder includes a Y/C separator coupled to a color demodulator. The Y/C separator receives and separates the video samples into luminance and chrominance components. The color demodulator receives and demodulates the chrominance component into color difference components.
Yet another aspect of the invention provides a video decoder for decoding a composite video signal. The video decoder includes an input sampling circuit coupled to a color decoder and a timing circuit. The input sampling circuit receives and digitizes the composite video signal with a first sampling signal to generate video samples. The color decoder receives and decodes the video samples to generate decoded video components. The timing circuit receives a reference clock signal and generates the first sampling signal. The timing circuit includes a phase lock loop (PLL) that receives the reference clock signal and a mode control signal and is configurable to operate in one of a number of operating modes indicated by the mode control signal.
In an embodiment, the plurality of operating modes is associated with different loop characteristics, and includes a fast mode and a slow mode. The fast mode is characterized by a fast loop response and the slow mode is characterized by a slow loop response. The PLL can be configurable to switch to the fast mode when an averaged PLL phase error exceeds a particular threshold.
Yet another aspect of the invention provides a video decoder for decoding a composite video signal. The video decoder includes an input resampler, a Y/C separator, a color demodulator, and an output resampler, all coupled in series. The input resampler receives and resamples input video samples with a first resampling signal to generate resampled video samples. The input video samples are generated by digitizing the composite video signal, and the first resampling signal is locked to color bursts of the composite video signal. The Y/C separator receives and separates the resampled video samples into luminance and chrominance components. The color demodulator receives and demodulates the chrominance component into color difference components. The output resampler receives and resamples the luminance and color difference components with a second resampling signal to generate output video components. The second resampling signal is locked to a line rate of the composite video signal.
The first and second resampling signals can be generated using first and second PLLs, respectively. Each PLL can be designed to operate in one or a number of operating modes, with each mode corresponding to different loop characteristics.
The Y/C separator can be implemented with a comb filter or an adaptive comb filter having a variable amount delay. To align the decoded picture, the output resampler can also be designed to generate output video components that takes into account a time difference between the start of a video line and the color burst phase of the video line.
The invention can be implemented in hardware, software, or a combination thereof.
The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.